Metallic vapor electrical discharge device



ay 1, 11934. p FREEDMAN 1,957,255

METALLIC VAPOR ELECTRICAL DISCHARGE DEVICE Filed May 22, 1929 2 Sheets-Sheet 1 mam May I, 1934 P FREEDMAN Q 1,957,255

METALLIC VAPOR ELECTRICAL DISCHARGE DEVICE Filed May 22, 1929 2 Sheets-Sheet 2 Patented May I, 1934 1,957,255 METALLIC VAPOR ELECTRICAL DISCHARGE DEVICE Paul Freedman, London, England Application May 22, 1929, Serial No.

In Great Britain May 25, 1928 1 Claim. (Cl. 250-275) The present invention relates to metallic vapor arc discharge devices,-in vitreous envelopes, containing one or more permanent gases, and which may be adapted to act as rectifiers, oscillators, sources of visible radiation, or, in particular, as sources of ultraviolet radiation, i. e. may be adapted to emit radiation of one or more wave lengths lying between the limits of 4,000 and 1,000 Angstrom units.

Devices constructed and operating according to the present invention, are so fashioned as to have high temperature, high pressure, metallic vapor arc discharge characteristics hitherto associated with the hereinbefore described metallic vapor arcs in silica containers, and moreover possess the relatively simple and inexpensive construction 'of metallic vapor arc discharge devices in glass containers.

According to the present invention, an are discharge in a given device having a permanent gas or gases therein, is caused to pass through a tubulated member of refractory insulating material, situated within thedevice and preferably but not necessarily, non-integral with the envelope of the device, having at least two apertures, for example, open at both ends, and having the region of the lower aperture immersed in the vaporizable metallic pool which acts as a source of the metallic vapor constituting the atmosphere in the region of the aforesaid arc discharge during the normal operation of the device, the construction being such that in the course of operation of the device, the generated metallic vapor expels the permanent gas or gases from within the aforesaid tubulated member, and, in consequence, while the atmosphere in the device, outside the tubulated member, during operation of the arc discharge, consists mainly of permanent gas or gases, that within the tubulated member consists only of the metallic vapor aforesaid; that is to say the tubulated member performs the function of confining the are region within it to metallic vapor issuing from the vaporizable pool aforementioned and serves to prevent the diffusion of molecules of the permanent gas within the device into the aforementioned arc region and also minimizes the diffusion of the metallic vapor from the aforementioned arc region into the gaseous atmosphere outside the tubulated member.

The vaporizable metallic pool, aforesaid, which is preferably of mercury, but may be of other suitable substance or substances, may constitute a vaporizable electrode of the arc discharge, or may act solely as a source of the hereinbefore mentioned metallic vapor, being raised for this purpose to an appropriate temperature in any suitable manner, as for example, by means of thermal energy, conveyed to it by radiation and conduction through the surrounding atmosphere,

from an electrically-heated filament of refractory conducting material, which filament may, or may not, be an electrode, or by means of thermal energy supplied by a resistance heater situated within the metallic pool and insulated from the said metallic pool by a sleeve or casing of suitable insulating material, such as silica or hard glass, or by a vitreous evacuated or gas-filled envelope, or by means of thermal energy supplied by a resistance heater situated outside the device, in close proximity to the metallic pool aforesaid.

The hereinbefore mentioned tubulated member of refractory insulating material may have a passage of uniform cross section or this passage may be enlarged or constricted in various places in a suitable manner. The tubulated member may, likewise, be either straight, or bent in a required manner and may be used either singly or, in the case where two or more are discharges pass, either simultaneously or successively, within the same device, may be joined in an appropriate manner to one or more tubulated members of refractory insulating material, employed for a similar purpose. The material of which the aforesaid tubulated member or members are composed is dependent on the precise purpose for which the particular device is intended. Thus, for example, if the particular device is designed to act as a source of ultraviolet radiation, the aforesaid tubulated member or members within it must be composed of silica or of a substance having similar thermal and optical properties. On the other hand, if the particular device is designed to act as a rectifier or an' oscillator, the aforesaid tubulated member or members may, advantageously, be composed of such substances as thoria, zirconia or porcelain.

In all cases, the device is so constructed that the pool or pools of vaporizable metal are automatically reconstituted by the recondensed metallic vapor.

The permanent gas or gases within the device may be at any convenient pressure, but it is preferable that their total quantity be such that, during the normal, continuous operation of the device the pressure within it is of the order of an atmosphere. The actual gas or gases employed depend upon the composition and temperature of operation of the vaporizable metal or metals, the metallic vapor and the refractory electrodes within the particular device, and the precise purpose for which the particular device is intended. Thus, for example, if the device is adapted to function as a source of ultraviolet, gases transparent to ultraviolet must be used. If the particular device is adapted to act as a rectifier or an oscillator, the vitreous envelope of the device may be advantageously composed of a glass commonly employed in electric lamp manufacture, as, for example, lead glass or soda lime glass.

On the other hand, if the device is adapted to act as a source of ultraviolet radiation, it is essential to employ a glass transparent to ultraviolet. Again, if the particular device is adapted to act as an ultraviolet source for tonic actino-therapy only, it would be most advantageous to employ a glass which would be transparent to wave lengths of from 4,000 to 2,960 Angstrom units, in particular to wave lengths between 3,000 and 2,960 Angstrom units, but. substantially opaque to shorter wave lengths. On the other hand, if the particular device is principally intended as a source of ultraviolet for germicidal purposes, it is essential to employ a glass transparent to ultraviolet of wave lengths shorter than 2,960 Angstrom units.

The vaporizable metal within the device is, preferably, mercury, but other metal or metals, such, for example, as sodium-potassium alloy, or cadmium-gallium alloy, may, in certain cases, be advantageously employed. The device may possess two or more electrodes of which some or all may be of refractory conducting materials and some or all of vaporizable metal or metals aforesaid. The refractory electrode or electrodes may be of various shape, size and composition, depending upon the precise purpose and condition of operation of the electrode or electrodes in question. Thus, for example, a refractory electrode acting as an anode, may be in the form of a rod or a sphere, plate, ring or block, or any other convenient shape, mounted on a stalk of refractory conducting material, and may be composed of tungsten, molybdenum, nickel, iron, ferro-nickel, carbon or any other suitable conducting metallic or non-metallic element or elements, but is preferably of molybdenum or tungsten. On the other hand, a refractory electrode acting as a cathode may be of tungsten or molybdenum, in the form of a sphere, plate, ring or block, or any other convenient shape, mounted on a stalk of refractory conducting material such as tungsten. It is particularly advantageous to employ as a cathode a fused tungsten bead mounted on a tungsten stalk. Again, a refractory electrode acting as a cathode may be composed of tungsten or molybdenum and one or more refractory metallic compounds, such that their inclusion tends to increase the electron emissivity of the said cathode, as, for example, thoria, and may be in the form of a rod, tube, plate, block or any other convenient shape, suitably mounted. Likewise, the refractory cathode may be in the form of a filament, straight, looped, crinkled or spiralized, adapted to be raised to the temperature, at which it is capable of adequate electron emission, by the passage of an auxiliary heating current, and may be composed of such materials as tungsten, tungsten coated with thorium oxide, or of platinum, platinum-iridium, or nickel, coated with oxide of calcium, barium and strontium, used either severally or admixed in suitable proportions. When a. filamentary cathode is employed, particularly if it be of the coated type, it may be advantageous to protect it from the bombardment of heavy positive ions by means of a grid, preferably a surrounding grid, of refractory conducting material, such as tungsten, molybdenum, or nickel, maintained either at the potential of the negative and of the filamentary cathode, or at a potential negative with respect to the negative end of the filamentary cathode afore- The essential function of the grid is to protect the active portion of the cathode from heavy positive ion bombardment; the cooler extremities of the cathode could be readily protected without the use of the grid simply by disposing them outside the arc stream. The relatively light positive ions of the permanent gas within the device do not exercise an appreciable disintegrating effect upon the active "portion of the cathode since the relatively high temperature of that portion reduces the oathode fall of potential very materially. The heavy mercury ions on the other hand would have a much more serious effect upon the active portion of the cathode and it is the function of the grid either to arrest such ions or to sufiiciently minimize the velocity with which they reach the active portion of the cathode.

A refractory electrode, which, in the course of starting or operation of the device, has to function both as an anode and as a cathode, must conform to limitations of shape and composition imposed on a refractory anode and on a refractory cathode, as hereinbefore described.

The temperature of a refractory anode in the device may vary from the highest value, compatible with a reasonably slow rate of evaporation of the anode material, and a temperature at, or in the neighborhood of, dull red heat. If it is desired to utilize the incandescent anode as a source of visible radiation, that is, to operate it at a temperature of bright incandescence, and to obtain, at the same time, a satisfactory anode life, it is necessary to employ, tungsten as anode material, and to employ an anode constructed in the form of a sphere, plate, ring or block, or any other convenient shape, mounted on a stalk of refractory conducting material, the most advantageous form being a fused tungsten bead On a tungsten stalk. The temperature of a vaporizable anode must be such as to permit the evolution from the surface of the said anode of the requisite quantity of metallic vapor. In the case of a refractory cathode, the temperature must be such as to permit an adequate electron emission and should, preferably, be as nearly as possible, such, that the sum of cathode loss per unit time by evaporation and cathode loss per unit time by disintegration produced by positive ion bombardment, is a minimum. If it is desired to employ the refractory cathode as a source of visible radiation, that is to operate it at a. temperature of bright incandescence, and, at the same time, to obtain a satisfactory cathode life, it is necessary to employ tungsten for the cathode material, preferably, though not necessarily, in the form of a fused tungsten bead mounted on a tungsten stalk, or in the form of a filamentary cathode, used in conjunction with the hereinbefore mentioned protective grid. In the case of a. vaporizable cathode, the temperature of the cathode hot spot must correspond to an adequate electron emission and, in addition, the cathode temperature must be such as to permit an adequate evolution of metallic vapor from the surface of the said cathode.

Various means as hereafter explained may be employed, to start the arc discharge within the device.

In the case of certain examples of the device, adapted to act as sources of visible or ultraviolet radiation, it may be necessary to adopt suitable means to counteract the deleterious effects of ultraviolet radiation upon the material composing the vitreous envelope of the device. It has been found that the deleterious effects of ultraviolet radiation upon certain glasses may be counteracted by subjecting the said glasses to a sufilcient quantity of infrared radiation. Accordingly, it is advantageous, in the case of such examples of the device, to generate a limited quantity of infrared radiation within the device. for the purpose set forth, in a manner.

described in my co-pending patent application Serial No. 365,038.

It will be understood that, in certain cases, electrical connection between the various electrodes of the present device and the external electrical circuits, may be effected by means of connections brought to one common stem, the leading in wires being sealed into the stem in any convenient manner, familiar to those skilled in the art, as described in my British Patent No. 313,727.

The connection leads or supports, extending from the leading in wires in the device towards the appropriate electrodes, are insulated, wherever necessary, from the conducting metallic vapor or ionized gas or gases within the device, by means of sleeves of insulating refractory material, such as silica, porcelain, mica, zirconia, thoria, alumina, or hard glass, threaded over them, or by means of exhausted or gas-filled vitreous envelopes. It will be understood that the connecting leads or supports themselves may be of any suitable dimensions andmaterial, except in so far as limited by the description of the present invention given above.

In order that the invention may be better understood, it will now be described with reference to the accompanying drawings in which:

Figs. 1 to 10 show ten methods of carrying the invention into effect.

Fig. 1 shows a type of the device which is constructed and operates in a manner identical with the device shown in Fig. 1 (and described) in my co-pending application Serial No. 365,038, except that the funnel employed to localize the cathode hot spot in the device described in the earlier specification is replaced by a tubulated member 11. The tubulated member 11 is composed of silica and consists of a dome-shaped portion 12 open at the bottom, a narrow tubular portion 13, a bulb 14 and a short narrow tubular portion 15, open at the top. When the lamp is not alight, the anode head 16 dips into the mercury cathode pool 17. Upon switching on the lamp, the solenoid 13 draws up the iron-filled envelope 19 and raises the anode bead 16 to the position central with the bulb 14. The are thus formed between the anode bead 16 and the mercury cathode portion 17 causes evolution of mercury vapor sufiicient to expel the permanent gas or gases from the tubulated member 11 so that the whole of the arc region and also the anode bead are in an atmosphere of mercury vapor only, while the atmosphere in the envelope 20 consists principally of permanent gas or gases and partly of mercury vapor. The peculiar advantage ofthe arrangement from the point of view of the use of the lamp as a source of ultraviolet radiation is that the atmosphere in the envelope 20 surrounding the arc-enclosing tubulated member 11 is, if the permanent gases are suitably chosen, highly transparent to ultraviolet, so that, since the arc discharge completely fills the portion 13 of the tubulated member 11, the absorption of the ultraviolet generated within the lamp is almost entirely governed by the transparency of the outer envelope 20 which can be made transparent over the desired region of wave lengths and opaque to other wave lengths.

It may be noted here that the above mentioned advantage may in general be realized in all lamps constructed according to the present invention.

Fig. 2 shows another application of the invention. The lamp is constructed and operated in a manner identical with that shown in Fig. 2 (and described) in my co-pending application Serial No. 365,038, except that in place of the funnel localizing the cathode hot spot, utilized in the device described in the earlier specification, a tubulated member 21 of silica is employed. The tubulated member 21 consists of a dome shaped portion 22 open at the bottom and a narrow tu bular portion 23, open and slightly flared out at the top. When the device is brought into operation, the bead 16 is withdrawn from the cathode pool 17 through and completely out of the narrow portion 23. In a lamp of this construction the mercury vapor completely fills the tubular portion 21 which encloses nearly the whole of the arc region, but the anode bead 16 is not surrounded by mercury vapor alone but by an atmosphere within the glass envelope 20, which, as in the preceding case, consists principally of permanent gas or gases and partly of mercury vapor. The advantage of this construction over that described with reference to Fig. 1 of the present specification, is that it is possible to operate the anode bead 16 at a considerably higher temperature than in the preceding case, since the eifect of the heat generated by the anode bead upon adjacent vitreous parts is very much less than in the first case and since, also, any deposition of disintegrated anode material, which must necessarily occur over a long period of operation, is distributed over a much wider area, so that the effective life of the device is thereby greatly increased.

Fig. 3 shows another type of the device constructed according to the present invention. This particular device is identical with the one shown in Fig. 5 (and described) in my co-pending application Serial No. 365,038, except that the arc discharge is caused to pass through the tubulated member 24 which may be of silica. The tubulated member 24 consists of a-dome portion 25 open at the bottom, a relatively wide tubular portion 26 and a spherical portion 27 having a mouth 28 of flattened slit-like section. The tubulated member 24 is supported in a manner identical with that employed for supporting the tubulated members in the two hereinbefore described instances. The device is adapted to function either on D. C., in which case the mercury pool 29 acts as an anode whilst the incandescent filament 30 acts as a cathode, or on A. C., in which case the device is self-rectifying and utilizes only one-half of the alternating current wave.

Fig. 4 shows another method of carrying the 35 invention into effect. This type of device bears a general resemblance to that shown in Fig. 3, with the following modifications. The tubulated member 31 which may be of silica, through which the arc discharge passes, is a tube of fairly large uniform cross section. The cathode 32 is in the form of a non-inductively conically wound spiral and is situated within a grid-like structure 33 which consists of a cylinder 34 open at the top and closed at the lower end by a wire gauze or network 35. Molybdenum or some other refractory metal is preferably used in the construction of this grid. As shown in the drawings, the grid is electrically connected to the filament and serves to protect it largely from the deleterious etfects of bombardment by heavy mercury ions. In order that the grid might be more effective in protecting the filament and that, at the same time, the current density and, consequently, the efficiency of the arc discharge may be maintained at a high value, it is in general advisable that the grid cylinder 34 should not make a tight fit with the tube 31. The device is capable of operating on direct or alternating current in a manner similar to that shown in Fig. 3.

The construction of the device shown in Fig. 5 is substantially the same as that shown in Fig. 4 (and described) in my co-pending application Serial No. 365,038, except that instead of the funnel for localizing the cathode hot spot employed in the earlier construction, the tubulated member 36 of silica, entirely enclosing the are discharge region and the anode bead 16 is employed, and that an external resistance element 37 is uti-' lized to obtain a transfer from a glow to an arc discharge in a manner described in my co-pending application Serial No. 365,039, as well as to assist in producing and maintaining a mercury vapor atmosphere within the tubulated member aforesaid. The tubulated member 36 comprises a dome-shaped portion 38 united by means of a graded glass joint 39 to the envelope 40 of the device. The top of the dome-shaped portion 38 is joined to a relatively narrow tubular portion 41 which terminates in a spherical bulb 42 having at its upper extremity a short narrow tubular projection 43, open at the top. The tubulated member 36 has four apertures, the uppermost 44 serving to admit the anode 16 and also to provide exit for the mercury vapor, the two lowermost apertures 45 serving to admit mercury within the dome 38 and the small aperture 46 at the side of the dome serving to admit the auxiliary electrode 47. The functioning of the device will be readily understood.

Fig. 6 shows another embodiment of the invention identical with that shown in Fig. 3 (and described) in my co-pending application Serial No. 365,038, except that instead of the funnel for localizing the hot spot utilized in the earlier device, the present device possesses a tubulated member 47 of silica, and that its anode 48 is adapted to be operated at a lower temperature than that of the earlier device. The tubulated member 47 is substantially identical with the tubulated member 13 shown in Fig. 1. The relatively low operating temperature of the anode 48 is necessitated by the relatively slow movement of the anode from the lower position to its final position upon application of the potential to the terminals of the lamp, it being inadvisable to move a very hot body slowly through the restricted portion of the tubulated member.

Fig. 7 shows another embodiment of the invention as applied to the case of a rectifier. The device possesses a large plate anode 49 which may be of any refractory metal, such as molybdenum or nickel, or may be composed of carbon. The cathode 50 is in the form of a fused tungsten bead mounted on a tungsten stalk. The tubulated member 51, through which the arc discharge passes, is of refractory insulating non-vitreous material, preferably zirconia or the like, and has a dome-shaped portion 52 open at the lower end and a relatively wide tubular portion 53, open at the top, and a narrow side branch 54 which serves to admit the cathode. The tubulated member 51 has a number of projections 55 and is supported by means of a lead 56 which is tied on to these projections. A quantity of mercury 57 is contained in a reservoir 58 at the bottom of the device. This mercury pool does not function as an electrode, but merely acts as a source of metallic vapor for the purposes described in the specification. The evolution of the mercury vapor is obtained by heating the mercury 57 by means of a resistance element 59 enclosed in a gas-filled vitreous envelope 60 and immersed in the mercury in the manner shown. In order that undesirable shorting within the device might be avoided, leads 56, 61, 62 and 63 are insulated in a manner shown, by refractory insulating tubing, hard glass being preferably used in the case of leads 56, 61 and 62 and preferably thoria or magnesia in the case of lead 63, and the cathode stalk entering the branch 54' of the tubulated member is, for the same purpose, insulated by the refractory insulating sleeve, preferably of thoria or magnesia, closely fitting the branch 54 of the tubulated member as shown in the accompanying drawings. The method of starting of the device is fundamentally the same as that described in my co-pending application Serial No. 365,039. When the device is in normal operation, the cathode and the greater portion of the arc region are surrounded solely by mercury vapor, while the anode 49 is surrounded by an atmosphere consisting principally of permanent gas or gases and partly of mercury vapor. The chief object of placing the anode outside the tubulated member is to enable the use of a large anode, which increases the efficient rectification, in combination with a relatively simple tubulated member 51.

Fig. 8 shows another embodiment of the invention as applied to the case of a lamp specifically designed for operating on alternating current and adapted to function as a source of visible or ultraviolet radiation. The device comprises a tubulated member 64- of silica having a dome-shaped portion 65 open at its lower end, a bent narrow portion 66, a spherical portion 67 having a small tubular branch 67, a narrow tubular portion 68, another spherical portion 69 and a short tubular portion 70 open at its upper end. Within the spherical portions 67, 69 of the tubulated member 64 are situated two similar electrodes 71, 72. These electrodes are in the form of fused tungsten beads and are mounted on tungsten stalks. The lamp comprises a vaporizable metallic pool 73, preferably of mercury, which, in the course of operation of the device, is heated by radiation and conduction through the gaseous atmosphere within the device by means of an independently heated filament 74, of tungsten, situated within the dome-shaped portion 65 of the tubulated member 64. The heating filament 74 is mounted on a stem 75 sealed into the lower portion of the vitreous envelope of the device. In order that the heating of the vaporizable metallic pool 73 may be more effectively carried out, the stem 75 and portions of the filamentary leads 76, 77 are completely immersed in the metallic pool 73, the leads 76 and 77 being insulated by means of glass sleeving integral with the stem, as shown. By means of this arrangement the filament 74 is brought to face directly the surface of the vaporiz'able metallic pool 73 to which it is therefore able to communicate a large proportion of its thermal energy. When appropriate electrical potential is applied to the terminals of the device, the filament 74 is rendered incandescent, communicates thermal energy to the vaporizable metallic pool 73, thereby causing an evolution of metallic vapor therefrom, and, at the same time,

ionizes the permanent gas or gases and also the metallic vapor within the dome-shaped portion 65 of the tubulated member 64. The ionized gases and metallic vapor rising upwards through the tubulated member 64, in their passage serve to provide a path between the electrodes 71 and 72 suficiently ionized to permit the formation of an electrical discharge between these electrodes, and a further modification of the atmosphere within the tubulated member 64, due to further evolution of metallic vapor from the vaporizable metallic pool, causes the discharge between the electrodes to assume normal arc characteristics. Additional heat is thus developed within the device and assists in the evolution of the metallic vapor from the vaporizable metallic pool, so that ultimately the tubulated member 6a is filled solely with metallic vapor, while the atmosphere within the device outside the tubulated member consists primarily of the permanent gas or gases within the device and partly of the metallic vapor aforesaid. Mechanical support for the tubulated member ca is provided by means of the projections 78 upon the stem 75, wires 79 being sealed into the said projections and tied to projections 80 situated at the lower edge of the dome-shaped portion 65 of the tubulated member 64. Insulat= ing sleeves 81, 82, 83 are provided to protect the electrode supports from the ionized gases within the device, the sleeve 81 being preferably of hard glass, whfle the sleeves 82 and 83 are preferably of insulating refractory material such as thoria. It is essential, from the point of view of efiiciency of the device, that the insulating sleeve 83 should form a fairly tight fit with the small tubular branch 67 of the tubulated member 64. It will be noted that the device furnishes an illustration of the use of a tubulated member having a suitably bent portion. In this particular case,- the bent portion 66 is provided in order that when the device is in operation, the arc discharge between the electrodes 71 and 72 may not extend to embrace the filament 74 which is in close spacial relation to the lower electrode.

Fig. 9 illustrates an application of the invention in the case of a device adapted to function as an alternating current lamp or as a full wave rectifier of alternating current. The device comprises two tubulated members 84. and forming a unitary structure with a common dome 86, the material of the tubulated members and the dome being dependent upon the functions which the device is intended to perform. Within the tubulated members 84 and 85 are situated anodes 87, 88 of refractory conducting material such as tungsten, molybdenum, nickel or carbon. The lower end of the dome 86 is immers d in the vaporizable metallic pool 89, preferably of mercury. Within the dome 86 is situated a filament preferably of tungsten. The filament is mounted in a manner precisely similar to that described with reference to the filament 74 in the form of the device shown in Fig. 8. The mechanical support of the tubulated structure within the device is also s'iinilar to that adopted in the form of the device shown in Fig. 8'. In this case, however, the filament 90 functions as a cathode. The manner of operation of the device will be readily understood by reference to the preceding descriptions. The figure also illustrates one well known convenient system of electrical connections. It is important to note that the anodes 87 and 88 must be operated at a relatively low temperature in order that a dash-over between them may be avoided.

Fig. 10 shows yet another application of the invention. The device shown in the figure is identical with that shown in Fig. 3 (and described) in my co-pending application Serial No. 365,039, except that the present device incorporates within it a ballast resistance 91, enclosed in a gas-filled vitreous envelope 92, and that the device also comprises a suitably shaped and supported tubulated member 93, of silica. The tubulated member 93 comprises a dome-shaped portion 94, open at its lower end, and a relatively narrow tubular portion 95 open at its upper end. The anode head 96 is situated immediately above the opening of the tubular portion 95 of the tubulated member 93. The dome-shaped portion 94 of the tubulated member has a small aperture 97 through which the auxiliary electrode passes. The manner of operation of the device will be clearly understood from the previous description and also from the specification of my co-pending application Serial No. 365,039.

It will be noted that devices shown in Figs. 3, 4, 5, 6, 7 and 10 are characterized by the fact that the electrical connection between various electrodes within the device and the external electrical circuit, is efiected by means of conductors all brought to one common stem, the leading in wires being sealed into this stem in accordance with my British Patent No. 313,727.

It will further be noted that in the case of devices shown in Figs. 6 and 10 ballast resistance units are incorporated within the device.

Referring to the types of devices specifically designed to function as sources of ultraviolet radiation, it will be understood that such sources may simultaneously function as sources of visible radiation. In fact there are many advantages attached to a lamp which functions simultaneously as a source of visible and ultraviolet radiation, particularly when such ultraviolet radiation does not substantially extend beyond 2,960 Angstrom units. In special cases, however, by using suitable glass for the envelope of the device, lamps which function as sources of ultraviolet but not of visible radiation may be constructed in accordance with the requirements of particular cases.

The invention is not limited to the precise forms or details of construction described, as these may be varied to suit particular cases.

What I claim and desire to secure by Letters Patent in the United States of America is:-

In a metallic vapor arc discharge device of the type comprising a vitreous envelope, 9. filamentary cathode electrode, an anode electrode, a tubulated member of refractory insulating material within the device surrounding the arc region. the tubulated member possessing at least two apertures and having the portion including the lower aperture immersed in the vaporizable metallic pool, one of the said electrodes being situated well within the tubulated member and the other electrode in close proximity to the aperture of the tubulated member which is not immersed in the vaporizable pool, and a grid electrode electrically surounding a part of the active portion of the cathode and having an open-work structure in the region intermediate between the anode and the cathode.

PAUL FREEDMAN. 

